In particular radio frequency (RF) applications it may be desirable to utilize a signal which is relatively low in amplitude as compared to other signals present in a communication system and/or which is present in a signal channel having other signals or noise energy very near thereto. Providing signal processing, such as signal down-conversion common in signal tuner applications, is particularly challenging with respect to such signals.
The OPENCABLE specifications from Cable Television Laboratories, Inc., for example, provide for three major RF functions via a common signal path, wherein the signal levels associated with one or more of the functions are significantly attenuated with respect to other ones of the signals. Moreover, the OPENCABLE signal channels are established very near other signal channels and/or occurrences of noise energy. To further complicate operation according to OPENCABLE specifications, the specifications establish relatively stringent signal quality requirements.
Specifically, the three major functions provided by the OPENCABLE specifications include a forward application terminal (FAT), a forward data channel (FDC), and a reverse data channel (RDC). The forward application terminal provides downstream analog National Television System Committee (NTSC) signals and high speed digital 64/256 quadrature amplitude modulation (QAM) signals in 6 MHz channel spacings. The forward data channel provides downstream low speed digital quaternary phase-shift keying (QPSK) signals in 1.0, 1.5, or 2.0 MHz channel spacings. The reverse data channel provides upstream low speed digital QPSK signals. Collectively, the forward data channel and the reverse data channel are often referred to as out-of-band.
The forward data channel of the OPENCABLE specification is provided for transmission within the 70 MHz to 130 MHz frequency band, and may be down-converted to a particular frequency, such as 36 MHz, for use by a terminal device, such as a set-top box. The forward application terminal channels are provided for transmission within the 54 MHz to 864 MHz frequency band, inclusive of the 70 MHz to 130 MHz frequency band (i.e., the 70 MHz to 130 MHz frequency band may be mixed use). Signals of the forward data channel are typically as much as 22 dB lower in amplitude than signals of the forward application terminal channels. Yet the output signal to noise and distortion (SINAD), or carrier to noise and interference (C/(N+I)), for the forward data channel is 20 dBc. These specifications lead to difficult and demanding signal processing requirements.
Specifically, forward data channel processing circuitry operating according to the aforementioned OPENCABLE specifications must accept signals from 70 MHz up to 130 MHz and provide down-conversion thereof to result in a signal to noise and distortion ratio of at least 20 dBc. However, at typical down-conversion frequencies, such as the aforementioned 36 MHz, images of the accepted frequency band are very near the frequencies of the accepted bandwidth. For example, the image frequency of 70 MHz, when down-conversion to 36 MHz is implemented, is 142 MHz (70+(2·36)=142). Accordingly, operation according to the specification requires down-conversion of frequencies as high as 130 MHz while needing to exclude images as low as 142 MHz.
Typical forward data channel processing circuitry which has been proposed includes a series of low pass filters provided in the signal path prior to a down-converter in order to reject noise energy including image frequencies, such as the aforementioned 142 MHz image frequency. For example, typical implementations utilize a down-converter having mixer oscillator circuitry outputting the down-converted signal as well as the image of the RF input. Therefore, to provide the desired signal quality at the output of such circuitry, it is necessary to filter out the image frequency entirely externally, such as by using discrete element capacitors and inductors in a discrete filter arrangement.
However, it is very difficult to build a sufficient low pass filter to adequately reject frequencies so closely spaced, such as with the 12 MHz separation between the high end pass frequency of 130 MHz and the 142 MHz image frequency. Specifically, in operation according to the OPENCABLE specification, required image rejection is set forth by the equation: (FDC to FAT adjacent channel amplitude)+(required signal to noise and distortion ratio)+(sufficient margin to all for other sources of noise and distortion) which, in practice, may establish an image rejection requirement of approximately 22 dB+20 dB+10 dB=52 dB. The difficulty in implementing circuitry to achieve such image rejection arises due to both the large amount of image rejection required, and the close spacing between the desired and image frequencies.
Building a low pass filter network suitable for meeting the image rejection requirements above, particularly from discrete components available today at costs conducive to mass production of circuitry acceptable in the marketplace in applications such as cable television terminal devices and/or broadband communication devices, is difficult. Moreover, in order to facilitate mass production of such circuits, it is desirable to avoid requiring manual alignment or adjustment of circuit attributes, as is commonly required with discrete component implementations.
One prior art solution with respect to providing such filtering has been to utilize a surface acoustic wave (SAW) filter rather than the multiple discrete component filter stages discussed above. However, the use of SAW filters is itself not without disadvantages. For example, SAW filters provide fixed pass-bands and, therefore, are useful only in situations where an out-of-band signal to be utilized is of a fixed frequency. Moreover, SAW devices are not capable of integrated circuit implementation using current technology and, therefore, must be provided as a discrete circuit component.
Accordingly, a need exists in the art for systems and methods that provide high signal quality output frequency conversion of signals relatively low in amplitude as compared to other signals present in a communication system and/or which are present in a signal channel having other signals or noise energy very near thereto. A further need exists in the art for such systems and methods to be adapted for mass production and/or deployment with little or no manual alignment or adjustment thereof.